1. Field of the Invention
This invention relates to a static magnetic field generating system. More particularly, it relates to a static magnetic field generating system which can be used suitably in combination with a nuclear magnetic resonance imaging (MRI) apparatus.
2. Description of the Related Art
Magnetic field generation means include a super-conducting magnet (SCM), a resistive magnet (RM), a permanent magnet (PM) and their hybrid magnet systems. These magnet systems generate magnetic fields inside a magnetic field generation space in accordance with their intended applications, but the magnetic flux leaks outside the required space depending on the intensity of the magnetic fields generated and the shape and size of each magnet system. This leakage magnetic field exerts an adverse influence on other instruments and appliances (e.g. electron beam application appliances such as color television, X-CT, magnetic tape application appliances, etc), people bearing a pace maker, a magnetic card, and so forth. A typical example of the adverse influences can be observed in a magnet used for a nuclear magnetic resonance imaging (MRI) apparatus. Recently, the field intensity has been increased in MRI apparatus to improve sensitivity, and a magnetic field as high as from 0.5 to 2.0 T (Tesla) has been employed. Particularly, the number of MRI apparatuses using 1.5 T has increased. In the MRI magnet 1 shown in FIG. 1, a human body 14, an NMR signal detection coil 13, a gradient magnetic field coil system 7 and a system 9 for improving a field homogeneity are accommodated in a magnetic field generation space 16. Therefore, this room temperature magnetic field generation space 16 has a large diameter of 800 to 1,000 mm. To generate a high magnetic field, the magnet becomes inevitably a superconducting magnetic system. In FIG. 1, a cylindrical superconducting magnet (SCM) is shown split longitudinally, and its internal structure is shown. A magnetic field Ho in a Z direction is generated in the internal cylindrical magnetic field space 16 by causing a permanent current Ip to flow through the superconducting coil group 1. To detect nuclear magnetic resonance (NMR) signals with a high sensitivity, the magnetic field Ho must be homogeneous over a wide range. Therefore, the superconducting coil group 1 is so designed as to attain .+-.5 ppm/40 cm dsv (diameter spherical volume), for example, and is finely adjusted by the shim system 9. The superconducting coil group 1 is positioned in a vessel 2 filled with liquid He, and this vessel 2 is encompassed by aluminum cylinders 3 and 4 kept at temperatures of 80 K. and 20 K., respectively, in order to reduce evaporation of liquid He. The cylinders 3 and 4 can be kept at the low temperatures by means of contact with a refrigerator 10. The inside of the magnet is kept at a vacuum in the order of 10.sup.-6 Torrs to reduce evaporation of liquid He.
In FIG. 1, reference numeral 5 denotes a magnet outer cylinder, 6 is a inner cylindrical bore, 11 is an 80 K. contact, 12 is a 20 K. contact, and 15 is a patient table.
FIG. 2 shows a closed loop which is formed when the lines of magnetic force of the cylindrical magnetic space 16 of the cylindrical superconducting magnet 17 described with reference to FIG. 1 leave one of the apertures of the space 16, pass through a loop in an external space as shown in the drawing, and then return to the other aperture of the space 16.
It can be understood from FIG. 2 that if the magnetic shield does not exist, a wide range is magnetically contaminated. When the diameter of the apertures of the magnet 17 is about 1.0 m and the field intensity inside the space 16 is 1.5 T (Tesla), for example, the contour line of the magnetic field 5 mT (5 Gauss), which is said to be a safe line for people bearing a pace maker, is shown in FIG. 3.
In other words, the surface of a Z axis ellipsoid of revolution which is .+-.12 to 13 m in the Z direction in the magnetic field generating direction of the magnet 17 and .+-.9 to 10 m in Y (vertical) and X (horizontal) directions is the contour line 20 of 5 mT. If the magnetic field generator is used without magnetic shielding, therefore, an extremely wide range must be kept off-limits, and various magnetosensitive appliances, as described above, cannot be used.
Therefore, a magnetic shield must be utilized.
Conventionally, the following magnetic shields have been employed.